In a gas turbine engine, different parts of the engine need to be lubricated. An oil system provides the lubrication oil to these different parts. The oil system includes a tank containing the oil to be supplied to the engine by an oil supply line. A scavenge pipe delivers that oil that has been used in the engine back to the tank. During lubrication of the engine, the oil becomes mixed with air, and when the oil is brought back to the tank air bubbles are also carried in. These bubbles can induce the production of foam in the tank, which may clog the air vent of the tank and as a consequence increase the pressure in the tank.